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RE: Historic Wood Structure; How to Dete

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Robert Rogers wrote:

. > I always thought grade & species were equally important with regard to
. > member strength....just the allowable bending stress on a Mixed Oak can
. > range from 725 psi (for a No. 2) to 1350 psi (Select Structural) per the
. > 1997 NDS supplement.....and I'm still a little confuse.......if I just
. > assume this value, what is my engineering basis for this assumption ?

. > I do like the idea regarding the increment borer.....and no, they did
. > not use one.....they mainly "looked" and "tapped" and did some drilling
. > with 1/8" bit & a portable drill....

First, let me say that a trip to the University library is in store for me 
this weekend to research ASTM standards on the full-size, in-grade testing of 
structural lumber.  Hence, what I say here is based on the previous testing 
program of small, clear wood specimens.

Testing of species was performed on small, 2" X 2" clear wood specimens.  
Grades were determined based on their "bending strength ratio," thus, for 
structural joists and planks (WWPA Grading Rules, 1981):

   Bending strength ratio     Grade
      65%                     Select Structural
      55%                     No. 1
      45%                     No. 2
      26%                     No. 3

Strength reducing characteristics for members were limited to those that 
supposedly would not affect the bending strength ratio below these 
percentages.  (Examples of calculating these effects are included in ASTM 
D245.)  Therefore, it is/was the basic strength of the species which 
determined the allowable stress in the various grades that caused me to say 
that species is more important than grade.

The UCBC is an ICBO (west coast) document that says DF No. 1 stresses can be 
used to evaluate existing wood structures.  It does not really address east 
coast hardwood construction.  Since the species is identified, I would 
use the allowable stresses for the species, not DF.  Since "mixed oak" is 
graded under the grading rules of NELMA, I would get a copy of their grading 
rule book, first, to see if there are members which would be below No. 1 
grade, and, second, to see if in-place grading would be necessary.  NELMA 
should also be able to provide a list of qualified visual graders.

Barry Welliver wrote:

. > The recommendation is to use the original allowable design values under 
. > the code in effect at the time of construction.

Yes, the UCBC does state that you can use the original allowable design 
values, but, *only if:*

   The load has not increased, *or,*
   The occupancy has not changed, *or,*
   Failure of a member has not occurred.

Otherwise, you have to use the current allowable design values.  In Robert 
Rogers' project, failure has occurred, e.g., shoring has been installed.

Structural failure:  When a structural member cannot support the loads or 
function as originally intended.

Roger Turk

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